(a) Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a transflective liquid crystal display.
(b) Description of the Related Art
Generally, a liquid crystal display (LCD) includes a pair of panels individually having electrodes formed on their inner surfaces, and a dielectric anisotropy liquid crystal layer interposed between the panels. Two polarizers are individually provided on the outer surfaces of the panels. In the LCD, a variation in the strength of an electric field generated by the electrodes changes the orientations of liquid crystal molecules in the liquid crystal layer, and the orientations of the liquid crystal molecules determine the polarization of light passing through the liquid crystal layer. At this time, the polarizers either pass or block the polarized light to produce white (or clear) or black (or dark) regions. As a result, a desired image display is realized.
LCDs are categorized as non-emissive displays, and in that respect, they do not produce any form of light. Accordingly, the LCDs utilize artificial light emitted from lamps of a backlight unit separately provided, or ambient light, as a light source.
Depending on the kinds of the light source used for image display, the LCDs are divided into three types: transmissive, reflective, and transreflective LCDs. In transmissive LCDs, the pixels are illuminated from behind using a backlight. In reflective LCDs, the pixels are illuminated from the front using incident light originating from the ambient environment. Transflective LCDs combine transmissive and reflective characteristics. Under medium light conditions, such as an indoor environment, or under complete darkness conditions, these LCDs are operated in a transmissive mode, while under very bright conditions, such as an outdoor environment, they are operated in a reflective mode. The reflective and transflective LCDs are commonly used in small and medium size display devices.
In a transflective LCD, there are transmission areas and reflection areas. In the reflection areas exterior light passes through the liquid crystal layer twice because of reflection, while in the transmission areas light emitted from the backlight provided behind an LCD panel assembly passes through the liquid crystal layer only once. Due to these characteristics, a gamma curve for the transmission areas and a gamma curve for the reflection areas do not coincide with each other. As a result, images are displayed differently in the transmission areas and the reflection areas.
There are some methods to solve the above-mentioned problem. One method is to form different thicknesses (i.e., cell gap) for the transmission areas and the reflection areas. Another method is to provide different voltages when the LCD operates in a transmissive mode that mainly uses the transmission areas and in a reflective mode that mainly uses the reflection areas.
However, the former method has some drawbacks. The manufacturing process becomes complex since a process to form a thick layer in the reflection areas is added. Also, problematic alignment of the liquid crystal layer, such as disclination and/or incidental images may occur due to a large stage difference generated at the boarders of the two areas. Furthermore, as the voltages applied to the reflective electrodes become larger, the luminance of the reflective electrodes becomes lower. Meanwhile, in the latter method, auxiliary capacitors are provided in the reflection areas to lower the voltages applied to the pixels. In this way, the problems of the former method can be resolved. However, the threshold voltages Vth of the two areas become different, so that a problem arises that the gamma curves of the two modes do not coincide with each other. Accordingly, the images displayed by the two modes are shown differently.
Meanwhile, the LCDs have a drawback that a standard viewing angle, based on the contrast ratio exceeding a predetermined level, is not very wide. The narrow standard viewing angle was a small matter since the transflective LCDs had been commonly used in the medium and small size display devices as mentioned in the above. However, recently, since the medium and small display devices are used in more applications, demands for the wide viewing angle are tending upwards in the field of LCDs.
Various methods have been proposed to enlarge the standard viewing angle of the LCD. A widely used method is to vertically align the liquid crystal layer to two panels and to form apertures or protrusions on the field generating electrodes. Another method is to control tilt directions of the liquid crystal molecules in various ways when an electric field is generated in the liquid crystal layer.
However, when these methods are applied to the transflective LCD that is commonly employed in the medium and small display devices, an additional process is required to form the apertures in the electric field generating electrodes. Further, it is not very easy to form the apertures in the pixels accurately because the pixels are too small, and even when the apertures are formed, the light efficiency, nevertheless, is not very high.